1. Field of the Invention
This invention relates to titanium and titanium alloy metal matrix composite billets produced by powder metallurgy for use as melt starting stock to produce metal matrix composite articles by casting.
2. Description of the Related Art
Titanium has many properties that make it an attractive material for high performance applications. For example, it has one of the highest strength-to-weight ratios of the structural metals, and will form a thin, tough protective oxide film making it extremely oxidation resistant.
Titanium and titanium alloy metal matrix composites have been developed for applications requiring enhanced physical and mechanical properties. By incorporating ceramic or intermetallic particles in a titanium alloy matrix, improvements in strength, modulus, hardness and wear resistance have been achieved. These particulate reinforced metal matrix composites are typically manufactured using powder metallurgical (P/M) methods. Examples of P/M processes are described in U.S. Pat. Nos. 4,731,115, 4,906,430, and 4,968,348, each of which is expressly incorporated herein by reference. To produce fully dense structural shapes, one preferred P/M process consists of blending pure titanium powder with appropriate ceramic or intermetallic materials in particulate form, together with alloying additions in either elemental or pre-alloyed powder form, then consolidating the blended powders in a controlled sequence: first, cold isostatic pressing, followed by vacuum sintering at elevated temperature and finally hot isostatic pressing. This CHIP process sequence results in a particulate reinforced metal matrix alloy in the form of a high density or fully dense solid, manufactured to a near-net shape.
Using this process, it is typically necessary to machine the P/M preform to achieve the final component shape and dimensions. Since machining requires a loss of starting material, and incurs significant costs associated with capital equipment, expensive tooling, labor and extended schedule, it is desirable to manufacture some titanium metal matrix composite components directly to the finished dimensions with little or no machining. Articles of titanium and titanium alloys may be produced most economically and repeatably to near net shape by casting.
Castings of titanium and its alloys are typically made by vacuum arc remelting (VAR) process, wherein a consumable electrode billet of the desired alloy composition is progressively melted into the liquid state by an electric current flowing across a voltage potential in the form of a plasma arc. The alloy melts from the electrode tip and collects in a molten pool contained within a crucible. To chemically isolate the highly reactive molten metal from the crucible walls and thus avoid a source of contamination, the crucible walls are actively cooled so that the first molten metal in the crucible forms a solidified layer or "skull." This skull ensures that the molten titanium does not come into direct contact with the crucible, but rather only contacts other titanium metal, thereby minimizing contamination of the final product. After enough molten metal has been collected in the crucible or the electrode billet has been consumed, the liquid metal is poured into a casting mold, wherein the molten metal solidifies and takes on the desired final component shape and dimensions.
Other vacuum melting methods, such as vacuum induction melting (VIM), may be similarly employed to render titanium and titanium alloys molten prior to casting.
The powder metal composite billets of this invention may also serve as starting stock for these melt processes when casting titanium metal matrix composite articles.